In general, an air suspension system serves to reduce a damage of a car body and provide a comfortable ride to a passenger by connecting a car axle and the car body with each other by pneumatic pressure to thereby properly reduce vibration or shock transferred from the surface of the road during drive, so as to adjust a buffer power according to drive conditions and provide smoother feeling of drive by adjusting pneumatic pressure.
Moreover, the air suspension system includes a spring tube mounted between a lower housing fixed to the car axle and an upper housing fixed to the car body. The spring tube performs a buffering action when compressed air is injected into the spring tube, and is formed in such a fashion that tube materials, which are stacked up in multiple layers, are inserted into a melting machine and are melted integrally.
U.S. Pat. No. 3,057,768 discloses a method of molding an air spring that an unhardened sleeve is outwardly pressed relative to an outer static mold by an inwardly expandable mandrill.
In U.S. Pat. Nos. 3,964,846 and 5,468,140, an air spring is cured by pressure of a mandrill acting from the outside to the inside, namely, toward the outer circumferential surface of a cartridge.
The spring tube melting machines according to the prior arts are manufactured with the purpose of easily manufacturing spring tubes, but have many problems in use.
First, when the mandrill presses the air spring while performing expansion and contraction, the surface of the air spring is formed unevenly because remaining air between the mandrill and the air spring cannot be discharged rapidly.
Second, when the mandrill which is used as consumable goods is replaced with a new one, there is an inconvenience to completely disassemble the spring tube melting machine.
Third, in order to form pyramid-shaped relief engraving portions on the air spring, because intaglio engraving portions, which is difficult to do cutting work, must be formed on the surface of the cartridge, it requires expensive prices to manufacture the cartridge.